Existing continuous carburizing furnace systems frequently include different sections or chambers for separating the various treatments employed in the carburizing processes-namely, heating, carburizing, diffusion, and hardening or equalize cooling. For example, U.S. Pat. Nos. 3,598,381 and 3,662,996 describe apparatus having separate furnace stages for heating, carburization, and diffusion of metal parts at selected temperatures and in different gaseous atmospheres for specific periods of time. In such systems, trays of parts are pushed or pulled, one after another, through each furnace in a fixed sequence, with each tray remaining in the same relative position in line throughout its passage through the system. Each part receives an identical process time.
Although the above-mentioned systems have been widely used for continuous runs of similar parts, they are not well-suited where it is necessary to process a variety of metal parts which require different cycle times and/or different times of quenching/cooling, and where it is desired to "manufacture-on-demand" a variety of parts so as to maintain low inventories.
Attempts have been made to provide greater flexibility of processing parts in furnace systems by the use of double rotary hearth carburizing furnaces, as disclosed in U.S. Pat. No. 4,622,006. U.S. Pat. No. 4,622,006 shows a rotary hearth carburizer and diffuser, providing a carburization and diffusion chamber separate from a conventional carburizing chamber. Systems of this type, don't allow parts to be processed in an efficient and smooth manner without some sort of scheduling.
For example, it is very likely that randomly loaded parts will need access to the same chamber at the same time. This results in at least one part having to spend an excessive amount of time in one chamber while waiting for access to the next chamber. Since the part is usually waiting in a hot chamber carburization continues, and the amount of carburization can be excessive, adversely affecting part quality. Additionally, the throughput of the furnace system can be affected, resulting in production loss.
The present invention is directed to overcoming one or more of the problems as set forth above.